The present invention generally relates to a method and apparatus for identifying failure sites on IC chips by a liquid crystal method and more particularly, relates to a method and apparatus for identifying failure sites on IC chips by a liquid crystal/glass substrate which is positioned in intimate contact with a top surface of an IC chip with the liquid crystal layer heated to a temperature just below its transition temperature such that any shorts or leakages in the IC chip will heat up the immediately adjacent liquid crystal material to pass its transition temperature and thus revealing a hot spot for identifying the failure site.
In the semiconductor fabrication technology, the capability and effectiveness of performing a failure analysis on a semiconductor chip package are very important. When an integrated circuit (IC) chip fails in service the nature and the cause for such failure must be determined in order to prevent the reoccurrence of such failure in similar products.
An IC chip is normally built on a silicon base substrate with many layers of insulating materials and metal interconnections. This type of multi-layer structure becomes more important in modern IC devices such as high density memory chips where, in order to save chip real estate, the active device is built upwards in many layers forming transistors, capacitors and other logic components.
When an IC device is found defective during a quality control test, various failure analysis techniques can be used to determine the cause of such failure. Two of the more recently developed techniques for performing failure analysis are the infrared light emission microscopy and the light-induced voltage alteration (LIVA) imaging technique. In the infrared light emission light analysis, an infrared light transmitted through a substrate silicon material is used to observe from the backside of an IC the failure mode of the circuit. For instance, at a magnification ratio of 100xc3x97, a failure point in the circuitry can be located. The LIVA imaging technique can be used to locate open-circuited and damaged junctions and to image transistor logic states. The LIVA images are produced by monitoring the voltage fluctuation of a constant current power supply when a laser beam is scanned over an IC. A high selectivity for locating defects is possible with the LIVA technique.
Another method that has become more common in failure analysis of IC chips is the scanning optical microscopy (SOM). The high focusing capability of SOM provides improved image resolution and depth comparable to conventional optical microscopy. It is a useful tool based on the laser beam""s interaction with the IC. The SOM technique enables the localization of photocurrents to produce optical beam induced current image that show junction regions and transistor logic states. Several major benefits are made possible by the SOM method when compared to a conventional scanning electron microscopy analysis. For instance, the benefits include the relative ease of making IC electrical connection, the no longer required vacuum system and the absence of ionizing radiation effects.
Even though the above discussed techniques are effective in identifying failure modes in IC circuits, the techniques require elaborate and complicated electronic equipment which are generally costly and not readily available in a semiconductor fabrication facility. It is therefore desirable to have available a method and apparatus that can be easily carried out without expensive laboratory equipment and thus the apparatus can be installed in any fabrication facilities. One of such methods is the use of a liquid crystal coating layer for the identification of failure sites in an IC chip. For instance, in a conventional method wherein a liquid crystal layer is used for the identification of failure sites, a liquid crystal material is frequently coated on top of an IC chip or an IC package. A typical set up is shown in FIG. 1.
As shown in FIG. 1 a typical liquid crystal detection apparatus 10 is provided. The apparatus 10 generally includes a heater 12 and an optical microscope 14. On a top surface 16 of the heater 12, an IC package 20 is positioned under the microscope 14 The IC package 20 may be a plastic quad flat pack (PQFP) or any other packaged IC device. The IC package 20, shown in FIG. 1, is completed with bonding pads 22 and bonding wires 24. In the middle portion of the package 20 are IC circuits that contain failure sites need to be identified by a liquid crystal method. In the conventional method, a liquid crystal material is first coated to the top surface 26 of the IC package 20. The IC package 20 is then positioned in top of the heater 12 which can be heated at a pre-programmed heating rate to a specific temperature. The IC package 20, together with the coated liquid crystal layer (not shown) is normally heated to a temperature just below the clear/opaque transition temperature of the liquid crystal material. For instance, a suitable temperature would be approximately between about 5xc2x0 and about 10xc2x0 below the transition temperature of the liquid crystal. After the IC package 20 is heated to such predetermined temperature, a pre-selected voltage is applied to the IC circuit through bonding wires 24. The IC circuit, upon receiving such a voltage, heats up at any short or leakage positions. A hot spot is thus generated at each of such locations. The liquid crystal material immediately adjacent, or contacting such hot spots therefore has its temperature raised above its transition temperature and transforms from an opaque state to a clear state. As a result, bright spots in the liquid crystal layer, i.e., on the IC package, show up to indicate the failure sites in the package.
Several drawbacks have been noted in the practice of the above described conventional method. First, a new coating of a liquid crystal material must be applied to each IC package or each IC chip to be examined under the microscope. The liquid crystal material once applied, is not reusable. i.e., cannot be transferred to another IC device. The coating and the drying process for the liquid crystal material for each IC package to be examined increase the complexity of the method. Secondly, the entire IC package, or the IC chip must be heated on the heater in order to bring the temperature of the liquid crystal coating layer to just below its transition temperature. Due to the large mass of the material to be heated, the exact temperature of the IC package is not only difficult to control but also requires a long time for heating to such temperature. Furthermore, the IC package requires a long time to cool once the temperature is heated passing its transition temperature. This further requires longer processing time
It is therefore an object of the present invention to provide a method for identifying failure sites on a defective IC chip that does not have the drawbacks and shortcomings of the conventional method of using liquid crystal material.
It is another object of the present invention to provide a method for identifying failure sites on a defective IC chip by utilizing a reusable liquid crystal apparatus such that each IC chip to be tested does not require a separate coating of a liquid crystal material.
It is a further object of the present invention to provide a method for identifying failure sites on a defective IC chip by utilizing a liquid crystal apparatus constructed by coating a liquid crystal material on a substantially transparent substrate.
It is another further object of the present invention to provide a method for identifying failure sites on a defective IC chip by utilizing a liquid crystal apparatus wherein a substrate equipped with heating elements is coated with a liquid crystal material to form a reusable liquid crystal apparatus.
It is still another object of the present invention to provide a method for identifying failure sites on a defective IC chip by utilizing a liquid crystal apparatus which is fabricated on a glass substrate heated by metal coatings of chromium, platinum or tungsten,.
It is yet another object of the present invention to provide a method for identifying failure sites on a defective IC package by mounting a liquid crystal device to an IC chip with a thermally conductive fluid medium thereinbetween to facilitate the heat transfer between the device and the chip.
It is yet another further object of the present invention to provide an apparatus for identifying failure sites on a defective IC package which consists of a substantially transparent glass substrate equipped with heating means and coated with a liquid crystal material, and a power supply capable of supplying a DC power to the substrate such that the liquid crystal material can be heated to a temperature just below its transition temperature when the apparatus is placed in contact with a defective IC chip.
It is still another further object of the present invention to provide an apparatus for identifying failure sites on a defective IC chip which includes a substantially glass substrate that can be heated and a liquid crystal coating thereon such that the apparatus can be mounted to an IC package with a thermally conducive fluid medium thereinbetween for improving the heat transfer between the apparatus and the chip.
In accordance with the present invention, a method and apparatus for identifying failure sites on a defective IC package or chip by utilizing a reusable liquid crystal device which can be mounted to various IC chips for detecting failure sites are provided.
In a preferred embodiment. a method for identifying failure sites on an IC chip can be carried out by the operating steps of first providing a substrate that is substantially transparent, then coating an electrically conductive material layer on a first side of the substrate while substantially retaining the transparency of the substrate, then coating a layer of liquid crystal on a second side of the substrate, then positioning the substrate. with the second side contacting a top surface of an IC device to be tested, then flowing an electrical current through the electrically conductive material layer and heating the liquid crystal material to a temperature not higher than its transition temperature, and then inputting a signal to the IC device and detecting hot spots formed under an optical microscope. The method may further include the step of providing a thermally conductive fluid between the substrate and the top surface of the IC device. The thermally conductive fluid may be conveniently a liquid glycerin for facilitating heat transfer between the substrate and the IC device. The substantially transparent substrate may be advantageously a glass substrate. The electrically conductive material layer may be coated on the substrate in a serpentine pattern or along an outer periphery of the first side of the substrate while substantially retaining the transparency of the substrate. The liquid crystal material coated has a clear/opaque transition temperature which may also be coated on the first side of the substrate on top of the electrically conductive material layer. The method may further include a step of flowing an electrical current between about 1 volt DC and about 50 volt DC through the electrically conductive material layer for heating the liquid crystal material to a temperature not higher than its transition temperature, and preferably to a temperature not more than 10xc2x0 C. below its transition temperature.
The present invention is further directed to a liquid crystal coated apparatus for use in failure site identification that includes substrate which is substantially transparent, a layer of electrically conductive material on a first side of the substrate, the layer of electrically conductive material has a thickness that does not affect the transparency of the substrate, a layer of a liquid crystal material on a second side of the substrate opposite to the first side, and a power supply capable of flowing an electrical current through the electrically conductive material layer and heating the liquid crystal material to a temperature not higher than its transition temperature. The liquid crystal coated apparatus can be used by positioning in contact with a surface of an IC device, optionally with a thermally conductive fluid medium sandwiched thereinbetween. The thermally conductive fluid sandwiched may be a liquid glycerin for improving heat transfer between the apparatus and the IC device. The power supply is capable of supplying a DC current between about 1 volt and about 50 volt. The liquid crystal material coated has a clear/opaque transition temperature between about 20xc2x0 C. and about 90xc2x0 C. The substantially transparent substrate on which the liquid crystal material is coated can be a glass substrate. The electrically conductive material coating layer can be formed of a metal such as chromium. platinum or tungsten in a serpentine pattern or along an outer periphery of the substrate. The present invention liquid crystal coated apparatus can be repeatedly used for detecting hot spots on IC devices.
The present invention is further directed to a method for detecting hot spots on a defective IC device which can be carried out by the steps of providing a substrate that is substantially transparent, coating an electrically conductive material on the first side of the substrate while substantially retaining the transparency of the substrate, coating a layer of liquid crystal material having a clear/opaque transition temperature on a second side of the substrate, contacting the substrate with the liquid crystal layer to a top surface of an IC device, flowing an electrical current between about 1 volt DC and about 50 volt DC through the electrically conductive material layer and heating the liquid crystal material to a temperature not higher than its transition temperature, and inputting a signal to the IC device and detecting hot spots formed under a optical microscope. The method may further include the step of providing a thermally conductive fluid sandwiched in-between the substrate and the IC device. The method may also include the step of removing the substrate from the IC device and mounting the substrate to another IC device to be tested. The method may heat the liquid crystal material to a temperature not more than 5xc2x0 C. below its clear/opaque transition temperature.